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A solution to this problem is to
eliminate copper in the reaction. In the last few decades, more
and more efficient palladium catalysts containing hindered
phosphane5 and NHC ligands6 have been developed working in
absence of Cu. Although the N-heterocyclic carbenes have been
used widely in organometallic chemistry and catalysis,7
Sonogashira coupling based on N-heterocyclic carbene is
relatively fewer comparing to phosphine based precatalyst. Most
of the copper-free Sonogashira reactions suffer from problems
due to the high catalyst loadings,8 and the use of unstable,
expensive and toxic phosphine ligands.5 Consequently, it is
necessary to develop air-stable, robust and well-defined Pdcomplexes
with inexpensive ligands.
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